The present invention relates to a fine carbon fiber having a specific structure and also relates to a production process and an application of the carbon fiber. More specifically, the present invention relates to a fine carbon fiber suitable as a filler for composite materials of resin, rubber or the like, a semiconductor material, a catalyst and a field emission material, and also relates to a production process thereof.
Carbon fiber is used in various composite materials because of its excellent properties such as high strength, high elastic modulus and high electric conductivity. With the progress of electronic technologies in recent years, carbon fiber is expected to be used as an electrically conductive resin filler for electromagnetic wave-shielding materials or antistatic materials or as a filler in a resin for use in an electrostatic coating by using not only the excellent mechanical properties of the carbon fiber, which have been heretofore utilized, but also the electrical conductivity of the carbon fiber or carbon material. Furthermore, the carbon material is expected to be used as a field emission material for a flat display and the like using of its properties such as chemical stability, thermal stability and fine structure.
Conventional carbon fiber is produced as a so-called organic carbon fiber which is obtained by heat-treating and carbonizing fiber such as PAN-(polyacrylonitrile), pitch- or cellulose-based fiber. In the case of using this carbon fiber as a filler for fiber reinforced composite materials, the carbon fiber is preferably reduced in its diameter or increased in its length, thereby enlarging the contact area with the matrix so as to elevate the reinforcement effect. Furthermore, for improving the adhesion to the matrix, the surface of the carbon fiber is preferably not smooth and is roughened to some extent by subjecting the surface of the carbon fiber to a surface treatment such as oxidation by exposure to air at a high temperature or coating or the like.
However, the organic fiber used as the starting material of this carbon fiber has a diameter of approximately from 5 to 10 xcexcm and therefore, the produced carbon fiber cannot have a small diameter and is limited in the ratio of length to diameter (i.e., aspect ratio). Under these circumstances, there is a demand for the development of carbon fiber having a small diameter and a large aspect ratio.
When resin is used for an automobile body or when resin, rubber or the like is used for an electronic device, the resin, rubber or the like is required to have electrical conductivity comparable to metal. Accordingly, there is a demand that the carbon fiber used as a filler material also has higher electrical conductivity so that the requirements demanded in various electrically conductive coating materials, electrically conductive resin and the like, can be satisfied.
In order to have higher electrical conductivity, the carbon fiber must be graphitized and thereby improved in electrical conductivity. To improve electrical conductivity, the carbon fiber is usually graphitized at a high temperature. However, even by graphitization, the carbon fiber cannot have electrical conductivity comparable to metal. If the amount of carbon fiber blended is increased to compensate for this insufficient electrical conductivity, the obtained composite material disadvantageously decreases in workability and mechanical properties. Therefore, it is necessary to further improve the electrical conductivity of the carbon fiber itself or enhance the strength by reducing the diameter.
With respect to the use as a field emission material, studies have heretofore been made on the field emission by the Spint method. However, the production process by this method involves many steps and although the carbon fiber used for the electron emitting part is conventionally processed to have a needle-like tip using Mo or the like, the chemical stability and the thermal stability are not sufficiently high as an electron emitting material of a display.
In the late 1980""s, studies have been made on vapor grown carbon fiber (hereinafter simply referred to as VGCF) of which the production process is utterly different from that of the organic fibers.
This VGCF is known to be obtained from the vapor-phase thermal decomposition of a gas such as hydrocarbon in the presence of an organic transition metallic catalyst, and a carbon fiber having a diameter of from hundreds of nm to 1 xcexcm is obtained.
For example, a method where an organic compound such as benzene is used as a starting material and an organic transition metal compound as a catalyst, such as ferrocene, is introduced into a high-temperature reaction furnace together with a carrier gas to produce VGCF on a substrate (JP-A-60-27700 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)), a method of producing VGCF in the free state (JP-A-60-54998), and a method of growing VGCF on a reaction furnace wall (Japanese Patent No. 2778434) are known.
According to these production processes, carbon fiber suitable as a filler material by having a relatively small diameter, an excellent electrical conductivity and a large aspect ratio can be obtained and in practice, carbon fiber having a diameter of approximately from 100 to 200 nm and an aspect ratio of approximately from 10 to 500 is mass-produced and used as an electrically conductive filler material in fillers for resin or in additive materials for lead storage batteries.
The VGCF is characterized by its shape and crystal structure. This fiber has a structure such that carbon hexagonal network surface crystals are stacked like annular rings to form a cylindrical shape, and the center part thereof forms a very narrow hollow moiety.
However, on a mass-production scale, VGCF having a small diameter of less than 100 nm cannot be produced.
Iijima, S., 1991, Nature, 354, 56, have discovered a multi-layer carbon nano-tube obtained from soot after the evaporation of a carbon electrode by arc discharge in a helium gas and this carbon fiber has a diameter smaller than that of VGCF. This multi-layer carbon nano-tube is a fine carbon fiber having a diameter of 1 to 30 nm, where, similarly to VGCF, carbon hexagonal network crystals are stacked like annular rings centered in the fiber axis and closed to form a cylindrical shape and the center part thereof has a hollow moiety.
This method using arc discharge is, however, not suitable for mass-production and not implemented in practice.
The vapor-phase process has a possibility of producing a carbon fiber having a large aspect ratio and a high electrical conductivity and studies are being made to improve this process with an attempt to produce a carbon fiber having a smaller diameter. U.S. Pat. No. 4,663,230 and JP-B-3-64606 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) disclose a cylindrical carbon fibril comprising graphite and having a diameter of about 3.5 to about 70 nm and an aspect ratio of 100 or more. The structure thereof is such that continuous layers of regularly oriented carbon atoms are disposed concentrically about the axis of the cylinder to form multiple layers, the C-axis of each carbon atom layer is substantially orthogonal to the cylinder axis, a thermal carbon coating deposited by thermal decomposition is not contained in the entirety, and the surface is smooth.
JP-A-61-70014 discloses a vapor grown carbon fiber having a diameter of 10 to 500 nm and an aspect ratio of 2 to 30,000, where the thickness of the pyrolytic carbon layer is 20% or less of the fiber diameter.
These carbon fibers all have a smooth surface, and therefore, are poor in adhesive property, wettability and affinity, and when used as a composite material, the surface of the carbon fiber must be treated, for example, by thorough oxidation. Furthermore, when used as a field emission material, the tip of the carbon fiber must be thinned.
An object of the present invention is to provide a fine carbon fiber capable of serving as a filler material having high electrical conductivity and a diameter of less than 400 nm, preferably from 2 to 300 nm, and exhibiting good adhesive property to resin or the like.
A further object of the present invention is to provide such fine carbon fibers on a mass-production scale.
Another object of the present invention is to provide a chemically and thermally stable field emission material having an excellent electron emission property and a long life.
The present inventors have discovered a new fine carbon fiber having a structure different from conventional carbon fibers, including the production process thereof. The present invention provides the following embodiments.
(1) a fine carbon fiber comprising cylindrical carbon sheets stacked to form a multilayer structure with the center axis thereof having a hollow structure, the fine carbon fiber having an outer diameter of 2 to 300 nm and an aspect ratio of 10 to 15,000, wherein at least one cylindrical carbon sheet layer among the multiple layers is folded at an end part of the carbon fiber and continued to another cylindrical carbon sheet and the folded and continued cylindrical carbon sheets form a cylindrical structure opened at the end part;
(2) the fine carbon fiber as described in (1), wherein the folded and continued cylindrical carbon sheets are present in the peripheral part of the multilayer structure;
(3) the fine carbon fiber as described in (2), wherein a cylindrical carbon sheet closed at the end part is present inside the cylindrical structure formed by the folded and continued cylindrical carbon sheets;
(4) the fine carbon fiber as described in (3), wherein cylindrical carbon sheets folded and continued at the end part to form a cylinder opened at the end part of the carbon fiber are present further inside the cylindrical carbon sheet closed at the end part;
(5) a fine carbon fiber having an outer diameter of 2 to 300 nm and an aspect ratio of 10 to 15,000, wherein the fine carbon fiber described in any one of the (1) to (4) occupies about 5% by mass or more of the fine carbon fibers;
(6) a fine carbon fiber having an outer diameter of 2 to 300 nm and an aspect ratio of 10 to 15,000, wherein the fine carbon fiber as described in any of (1) to (4) occupies from about 5 to about 90% by mass of the fine carbon fibers;
(7) a fine carbon fiber having an outer diameter of from 2 to 300 nm and an aspect ratio of from 10 to 15,000, wherein when observed through a transmission electron microscope, the fine carbon fiber described in any one of the (1) to (6) occupies from about 3 to about 80% by volume in the fine carbon fibers;
(8) the fine carbon fiber as described in any one of the (1) to (7), wherein the fine carbon fiber is vapor grown carbon fiber;
(9) the fine carbon fiber as described in any one of the (1) to (8), wherein the carbon fiber comprises a boron atom;
(10) the fine carbon fiber as described in any one of the (1) to (9), wherein carbon atoms of the carbon fiber are partially displaced by boron atoms;
(11) a process for producing the fine carbon fiber described in any one of the (1) to (10), comprising heat-treating fine carbon fiber having an outer diameter of 2 to 300 nm and an aspect ratio of 10 to 15,000 and having a multilayer structure formed by cylindrical carbon sheets stacked one on another, with the center axis having a hollow structure;
(12) the process for producing the fine carbon fiber as described in (11), wherein the heat-treatment temperature is from about 2,000 to about 3,500xc2x0 C.; and
(13) the process for producing the fine carbon fiber as described in (11) or (12), wherein a boron compound is mixed with fine carbon fiber having an outer diameter of from 2 to 300 nm and an aspect ratio of from 10 to 15,000 and having a multilayer structure formed by cylindrical carbon sheets stacked one on another, with the center axis having a hollow structure, and the mixture is heat-treated.